Future network systems may include long-haul optical communication systems, interconnection technologies, two dimensional optical processing, optical computing and others. Semiconductor devices, such as lasers and photodetectors, are already an integral part of optical fiber communication systems. In conjunction with fibers, other semiconductor devices, such as modulators and optical switches, are also likely to be incorporated into the network systems.
Unfortunately, the utility of many such semiconductor devices is hampered by their high fiber insertion loss which at least partially arises from a fundamental mismatch between a typical single-mode fiber with a relatively large cylindrical core and, thus, a large circular modal input or (output) area, and semiconductor devices having smaller modal output (or input) areas and eccentricity ratios greater than 1:1. Losses which arise in coupling light between optical fibers and these devices include those arising from the mismatch of the symmetry of the two modes (circular versus elliptical) as well as the mismatch of the average modal area.
In the past, symmetric hemispherically and hyperbolically shaped microlenses have been fabricated on the end of an optical fiber by means of a pulsed laser beam. See U.S. Pat. No. 4,932,989, issued to H. M. Presby on Jun. 12, 1990 and U.S. Pat. No. 5,011,254 issued to C. A. Edwards and H. M. Presby on Apr. 30, 1991. Such microlenses afford relatively high coupling efficiency for devices, such as lasers, having a symmetric modal output, that is, for devices whose output beam profiles are circular or have ellipticity ratios close to 1:1 i.e., where the divergence of the output beam of the laser is the same or substantially the same along axes parallel and perpendicular to the junction plane of the laser. Use of hyperbolically shaped microlensed fibers has led to greater than 90 percent coupling efficiencies between optical fibers and devices having symmetric modal output. However, the modal asymmetry exhibited by many semiconductor devices requires, for good coupling efficiencies, asymmetric microlenses. There are many lasers which have an elliptical beam structure with ellipticities from about 1:1.5 and even higher, emanating from the laser facet. Use of symmetric microlenses for coupling elliptical light beams to fibers, led to significant decrease in the coupling efficiencies. For example, for such semiconductor devices as laser diodes with reasonable modal asymmetry, e.g. 1:2.5 to 1:3.5, fiber coupling efficiencies of up to 50 percent can be obtained with symmetric microlenses, with 25 to 35 percent being more typical. Since about half of the laser output is not utilized, the laser has to be run at higher currents to yield the same coupled power into fiber than a more efficient coupling scheme could give. Running the laser at higher currents results in greater heat to be dissipated. For example, when the coupling efficiency is at 50 percent, the laser thermal power dissipation is four times greater than at 100 percent coupling efficiency. This affects long-term stability and reliability of the lasers and presents a major obstacle in the development of uncooled laser diode technology. For modulators and switches, where from a system design viewpoint an insertion loss of less than 0.5-1.0 dB is desired, the situation could be more serious. A higher, e.g. 3 dB, insertion loss decreases signal to noise ratio and increases system complexity.
Attempts to increase coupling of fibers to elliptical beams with non-symmetric lenses have been reported in the form of an externally mounted cylindrical lens and a wedge-shaped fiber endface. See M. Saruwatari et al. "Semiconductor Laser to Single-Mode Fiber Coupler," Applied Optics, Vol. 18, No. 11, 1979, pages 1847-1856 and V. S. Shah et al. "Efficient Power Coupling from a 980 nm, Broad Area Laser to a Single-Mode Fiber Using a Wedge-Shaped Fiber Endface", J. Lightwave Technology, Vol. 8, No. 9, 1990, pages 1313-1318. In the former case the coupling is effected by means of a lens and a cylindrical rod placed between a laser and an optical fiber, and in the latter case an end of the fiber is provided with an enlarged cylindrical portion terminating in a wedge-like shape which approximates a cylindrical lens. In the latter case, a coupling efficiency of 47 percent was obtained. Clearly, what is required for optimum coupling between a device with an elliptical light beam output (or input) area and an optical fiber is a lens which would transform the elliptical beam output of the device to match the circular single-mode fiber mode profile and vice versa.